(57) @ An assay for the free portion of an analyte, present in a biological fluid which also contains the analyte bound to natural binders, comprises incubating a sample of the fluid with a specific binder for the analyte and an antibody to the specific binder whereby the analyte and the antibody are caused to compete for binding sites on the specific binder. The antibody is preferably an anti-idiotype antibody to the specific binder. One of the specific binder and the antibody is labelled, whereby the proportion of the labelled reagent bound to its binding partner can be measured and used to determine the free analvte concentration in the samole.